Fix posix prio process (same as MR458 with fixed commits)

During a test compilation of ZSTD I discovered that it tries to use a constant from sys/resource.h named PRIO_PROCESS, but relibc had this constant defined as PRIO_PROGRESS with same value and context. This change is a fix of this typo, I couldn't find anything else in the codebase using this constant with the old name so existing sources shouldn't be affected, relibc and an OS image was built with the change successfully. This MR is the same as MR 458 but I accidentally made some mistakes with the commit hashes so the easiest solution was to create a new MR to keep the working tree clean. This also includes the tests discussed in MR 458.